Lunar Maria

What it is

These are the Moon’s maria or, in the singular, lunar mare (Latin word for sea), which cover about 17% of the lunar crust. The image shows the two major environmental features of the lunar surface, the bright and heavily cratered highlands (or terrae) and the smooth dark maria. Since, these plains are darker than the bright highlands the early observers interpreted them as water seas.

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Currently, it is known that these vast dark plains are formed by basalts from volcanic eruptions, which are a widespread volcanic rock on Earth. However, the lunar basalts are completely devoid of water and contain few volatile elements. The Moon’s basaltic rocks are fine-grained composed mainly by iron, magnesium and titanium. The maria appear dark due to their high iron and titanium content that lead to about half of the highlands albedo. The highlands are formed by anorthosite that is a porous rock composed mainly of plagioclase feldspar. The lunar basaltic lava were far more fluid than Earth’s lavas and thus formed smooth basins like oceans, as shown in the photography.

Lunar basalts are less porous than the anorthosites, so highland’s craters are generally deeper than maria’s ones. However, craters may be degraded by the cumulative effects of small impacts. Currently the evolution of the lunar landscape is dominated by the occasional impact craters, since in the absence of an atmosphere there is no erosive agents such as winds and rain. The relatively low number of craters in the maria is an indication of young ages.

What scientists think about it

The lunar maria have aroused the curiosity of people of different places and cultures along the time. The man in the Moon, the Moon rabbit, and Saint George killing the dragon are examples of lunar pareidolia. From the point of view of science, lunar samples from Appolo and Luna missions provide important, but insufficient data to disentangle the thermal history and evolution of the Moon. For instance, most lunar mare has not been sampled and data for these mare basalts are derived only by remote sensing, impact crater counts, and crater degradation. Studies of ages and chemical composition of the basaltic lunar maria may provide clues on the Moon’s internal formation and evolution. The acquired knowledge can be extended to develop evolutive models for distant moons and planets. In addition, geologic maps may be used in the preparation of future missions.

Why one should know about it

The highlands formed very early while the Moon’s surface was molten. In the early phase of the Moon formation its outer layers, heated by the kinetic energy of impacting bodies and the decay of radioactive elements, were molten forming a global magma ocean. The densest minerals such as iron and magnesium silicates crystallized and sank after the cooling of the magma ocean. Then, the less dense minerals such as anorthositic plagioclase feldspar crystallized and floated to the magma surface forming an anorthosite crust. Anorthosite is the most abundant and oldest rock on the lunar crust.

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An early basalt can be found in the highlands crust. Since these basalts present high concentrations of potassium (K), rare earth elements (REE), and phosphorous (P) it is called KREEP. Unfilled craters also present KREEP basalt, which suggest that the basins formed first by large impacts and subsequently were filled by iron rich mare basalts erupted directly from the melted mantle to the surface through the fissure in the lunar crust. There are evidence that most large impact basins were excavated in the lunar surface mainly during the heavy bombardment of the inner solar system or lunar cataclysm occurred in the range of 4.1 to 3.8 billion years ago, in the Early Imbrian epoch. On the other hand, most lunar mare basalts last from 3.8 until 3.2 billion years ago. The lower density of impact craters also suggest that the maria are younger than the heavily cratered highlands. The maria distribution on the lunar surface shows an asymmetry with most of them located in the nearside. A possible explanation is that the crust is significantly thinner on the nearside making it more easy for magma to reach the lunar surface. This asymmetry is often attributed to the South Pole–Aitken basin (the oldest lunar basin) ejecta. The absence of recent significant tectonic activity suggest that the Moon dissipated its internal energy and is currently dynamicaly inactive.